What is an Arduino ?
Arduino is a tool for making devices that can sense and control the physical world. Arduino can be used to develop interactive objects, taking inputs from a variety of switches or sensors, and controlling a variety of lights, motors, and other physical outputs. Arduino projects can be stand-alone, or they can be communicate with software running on your computer. The boards can be assembled by hand or purchased pre-assembled as do-it-yourself kits; the software for uploading programs into the board (open-source IDE) can be downloaded for free.
Why Arduino ??
There are many other microcontrollers and microcontroller platforms available for
physical computing. Parallax Basic Stamp, Netmedia's BX-24, Phidgets, MIT's
Handyboard, and many others offer similar functionality. All of these tools take the
messy details of microcontroller programming and wrap it up in an easy-to-use
package. Arduino also simplifies the process of working with microcontrollers, but it
offers some advantage for teachers, students, and interested amateurs over other
Simple, clear programming environment
Open source and extensible software
Open source and extensible hardware
Watch this video tutorial from MIT to have a better idea :
Further tutorials available here : http://arduino.cc/en/Tutorial/HomePage